NL9202295A - POWER SUPPLY AND CONTROL CIRCUIT FOR USE IN A POWER SUPPLY. - Google Patents

Description

Power supply circuit and control circuit for use in a power supply circuit.

The invention relates to a power supply circuit (VS) comprising input terminals (1, 2) for receiving an input voltage (Vin) and output terminals (3, 4) for outputting an output voltage (Vout) containing in parallel coupled to the input terminals of the power supply circuit a series circuit of a primary winding of a transformer and a switch, a secondary winding of the transformer coupled to the output terminals of the power supply circuit and a control circuit with an input coupled to the output terminals of the power supply circuit and with an output for connecting to the switch delivering a control signal.

The invention also relates to a control circuit for use in such a power supply circuit.

Such a power supply circuit is known from European patent application EP-A 380033. This known power supply circuit comprises a pulse width modulation control in which the control circuit receives two input signals. A first input signal for determining the starting moment of a new pulse, and a second input signal for receiving a signal related to the output voltage.

This known power supply circuit has the drawback that the power supply circuit does not work efficiently and the transformer can become saturated.

The object of the invention is to eliminate the above drawbacks.

To this end, a power supply circuit according to the invention has the feature that the control circuit is adapted to receive a feedback signal depending on the power to be supplied to the output terminals by the power supply circuit and is adapted to receive a demagnetizing signal at a further input, which control circuit operates has a first mode in which the control circuit drives the switch with the control signal, the frequency of the pulses of the control signal being dependent on the feedback signal and in which the control signal is delayed by means of a delay circuit under the influence of the demagnetizing signal, and a second mode with a predetermined fixed frequency and the feedback signal determines the duration of the pulses.

This ensures that the supply circuit functions optimally in normal operation as well as in standby. For normal operation, it is very important that the transformer is fully demagnetized before the switch is brought back into conduction, otherwise the transformer will become saturated, causing the transformer to malfunction and the energy to be transported to the secondary side. In stand-by, the demagnetizing signal is irrelevant since the transformer will not saturate. However, in standby, it must be ensured that the switch is at least the minimum collision in conductivity. Because the load in standby is much less than in normal operation, the switch only needs to conduct for a much shorter time. To prevent the minimum collision from being reached, the frequency of the control pulses of the switch in standby is set to a fixed frequency (usually lower frequency).

It stated that the European patent application EP-A 420997 discloses a supply circuit in which the control circuit receives an demagnetizing signal at an input. However, no switching between normal operation and standby is carried out in this power supply circuit. An exemplary embodiment of a power supply circuit is characterized in that the control circuit operates at a feedback signal less than a predetermined value in the first mode and at a feedback signal greater than a predetermined value in the second mode.

These and other aspects of the invention will be clear and elucidated on the basis of the exemplary embodiments described below.

In the figures shows:

Figure 1 shows a first exemplary embodiment of a power supply circuit according to the invention,

Figure 2 voltage-time diagrams for the different signals,

Figure 3 shows an exemplary embodiment of a demagnetizing detection circuit for a power supply circuit according to the invention, and

Figure 4 shows an exemplary embodiment of a part of an oscillator circuit.

Figure 1 shows an exemplary embodiment of a switched supply circuit VS with input terminals 1 and 2 to which an input voltage Vi is applied (for example from a mains-connected rectifier). Over the input terminals 1,2 there is a series connection of a primary winding Lp of a transformer T and a switch S. The switch S can be, for example, a bipolar high-voltage transistor or a MOS field effect transistor. A series connection of a diode Do and a storage capacitor Co is connected across a secondary winding Ls of the transformer. Output terminals 3 and 4 of the supply circuit are connected across the storage capacitor for supplying an output voltage Vo. A load Rb can be connected across the output terminals 3, 4. The switch S is brought in and out of conduction by means of control pulses P from an output 9 of a control circuit 6, the on-time (conduction time) of the switch being determined, inter alia, by the load connected to the supply circuit. At an input 8, the control circuit receives a feedback signal Vb which is a measure of the power to be supplied to the load by the supply circuit. At a second input 7, the control circuit 6 receives a signal 1d which indicates whether the transformer T is completely demagnetized. The second input 7 is connected to an additional winding Ld on the transformer for detecting whether or not it is demagnetized. The control circuit 6 includes a load detecting circuit 63 which is coupled to the input 8 for receiving the feedback signal Vb. The control circuit further includes a demagnetizing detection circuit 61 coupled to the input 7 for receiving the signal 1d. The demagnetizing detection circuit determines on the basis of this signal whether or not the transformer is completely demagnetized. It is very important that the transformer be fully demagnetized before the switch S is brought back into conduction. If the switch is turned on before the transformer is fully demagnetized, the transformer will become fully saturated over time and will no longer work as a transformer, as a result of which the supply circuit can no longer supply the requested output voltage. Furthermore, if the transformer becomes saturated or operates near saturation, the power circuit will consume (dissipate) much more energy than if the transformer is not operating in saturation.

The control circuit further includes an oscillator circuit 65 for generating and outputting a control signal to the switch. This oscillator circuit contains switching means, which switching means switches the oscillator operation between two different modes. A first mode in which the oscillator frequency is variable and the frequency is controlled depending on the load Rb. However, in order to prevent the transformer from becoming saturated, the oscillator circuit 65 of the demagnetization detecting circuit 61 receives a demag signal Dd, which demag signal in this mode prevents the oscillator circuit from sending a further control pulse to the switch. before the transformer T is completely demagnetized. The oscillator circuit 65 operates in this first mode as long as the load on the power supply circuit is high enough, i.e. during normal operation.

If the load falls below a predetermined value, the oscillator circuit switches to a second mode, in which second mode the oscillator supplies control pulses with a fixed frequency and in which the demag signal Dd is no longer considered. At a low load, the transformer can never become saturated, so the demag signal need not be considered.

By allowing the control circuit to operate in two different modes depending on the load, a power supply circuit is obtained which operates efficiently in both modes, and thus has minimal dissipation.

Figure 2 shows voltage-time diagrams for the two modes. Figure 2a shows the voltages for a low load and figure 2b shows the same voltages for a high load.

Figure 2a shows the voltage Vs across the switch S, the demagnetizing signal Id, the oscillator sawtooth Os, from which the control pulses P are derived and the demag signal Dd. As described above, at a low load (for example during stand-by) the oscillator circuit 65 does not look at the demag signal Dd, in Figure 2a the demag signal is already low before the oscillator circuit sends the next control pulse to the switch S will steer. The demag signal has no influence on the starting moment of the rising edge of the oscillator sawtooth Os.

Figure 2b also shows the voltage Vs across the switch S, the demagnetizing signal Id, the oscillator sawtooth Os, from which the control pulses P are derived and the demag signal Dd. Here, a high load Rb is assumed and it can be seen that the rising edge of the oscillator sawtooth (and thus the next control pulse to the switch) does not start until the demag signal Dd has become low, and so the transformer is first completely demagnetized before the switch S is brought back into conduction.

Figure 3 shows the demagnetization detecting circuit 61 in more detail. As described above, the input of the demagnetization detection circuit is connected to the input 7 of the control circuit on which the demagnetization detection circuit receives the demagnetization signal 1d. The input 7 is connected to a clamp circuit 611 for vaporizing the demagnetizing signal. An output of the clamp circuit is connected to a non-inverting input of a comparator 613, which comparator on the inverting input receives a reference voltage Vref of, for example, 65 mV. The output of the comparator 613 is connected to a reset input R of a flip-flop 615. At the set input S, the flip-flop receives a signal from the oscillator circuit (high at a rising edge of the oscillator sawtooth Os (see Figure 2) and low with a falling edge of the oscillator sawtooth). As long as the demagnetizing signal 1d is high, the input R of the flip-flop has a digital high signal, so that the output Q of the flip-flop will be low. The comparator output is also connected to a first input of an AND circuit 617, a second input of the AND circuit is connected to the output Q of the flip-flop. The AND circuit outputs the demag signal Dd (to the oscillator circuit) at the output. In this way it is ensured that as long as the demagnetizing signal 1d is high and the transformer is not yet fully demagnetized, the oscillator circuit in the first mode (i.e. at high load) cannot yet supply control pulses to the switch.

Figure 4a shows an embodiment of a part of the oscillator circuit 65 in more detail. A power source 651 is in series with a switch S2 and a capacitor Ct, the power source charging the capacitor when the switch is closed. A series connection of a switch S3 and a current source 652 for discharging the capacitor is connected to the connection point of the switch S2 and the capacitor Ct. The junction of the switch S2 and the capacitor Ct is also connected to an input of an operational amplifier acting as a Schmitt trigger, the output of the amplifier being connected to a control input of the switch S3 for opening the switch again. The output of the amplifier is also connected via an inverter 654 to a set input of a flip-flop 655. On a reset input, the flip-flop receives the inverted demag signal. An inverting output of the flip-flop is connected to an input of a NAND circuit 656. On a second input, the NAND circuit 656 receives the inverted demag signal. The output of the NAND circuit is connected to an input of an AND circuit 657. A second input of the AND circuit is connected to the inverter 654. The output of the AND circuit 657 outputs a signal Vosc which also serves as a switching signal for switch S2. In this way, the rising edge of the oscillator sawtooth (see Figure 4b) is delayed when the demag signal Dd is high (and thus the inverted demag signal low). As described above, the demag signal is applied only during the high load mode to delay the rising edge of the oscillator sawtooth if necessary. In order to work with a variable frequency in the second mode without the demag signal having any influence, the reset input of the flip-flop 655 must be connected to a digital layer in that situation. This is not shown here but is clear to those skilled in the art.

In Figure 4b, the demag signal Dd and the oscillator sawtooth are shown plotted over time.

It will be clear that the various elements of this power supply circuit can be designed in various alternative ways without departing from the essence of the invention. For example, the demagnetizing detection circuit can be adapted in all kinds of analog or digital ways without the operation of which differs substantially from the operation described above. The oscillator circuit can also be adjusted in all kinds of ways, for instance with a fixed frequency instead of a variable frequency in the first mode.

Claims (4)

1. Power supply circuit (VS) provided with input dampers (1, 2) for receiving an input voltage (Vin) and output dampers (3, 4) for supplying an output voltage (Vout), comprising in parallel coupled to the input dampers of the power supply circuit a series connection of a primary winding of a transformer and a switch, a secondary winding of the transformer coupled to the output terminals of the supply circuit and a control circuit with an input coupled to the output terminals of the supply circuit and with an output for supplying a switch control signal, characterized in that the control circuit is adapted to receive a feedback signal depending on the power to be supplied by the supply circuit to the output terminals and is adapted to receive a demagnetising signal at a further input, which control circuit operates in a first mode has a predetermined fixed frequency e and wherein the feedback signal determines the duration of the pulses, and a second mode in which the control circuit drives the switch with the control signal, the frequency of the pulses of the control signal being dependent on the feedback signal and the control signal being delayed by means of a delay circuit under the influence of the demagnetising signal. Power supply circuit according to claim 1, characterized in that the control circuit operates in the first mode at a feedback signal less than a predetermined value. Power supply circuit according to claim 1, characterized in that the control circuit operates in the second mode at a feedback signal greater than a predetermined value. Control circuit for use in a power supply circuit according to claim 1, 2 or 3.